PEDOT:PSS-Coated Screen-Printed Graphene–Carbon Ink-Based Humidity and Temperature Sensor

This work presents a multifunctional humidity and temperature sensor, fabricated on a flexible polyvinyl chloride (PVC) substrate using facile screen printing and drop casting. The sensor comprises screen-printed graphene–carbon (G–C) layer coated with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The sensor displays good sensing performance (~1%/%RH) toward wide humidity range (25%RH–90%RH) at room temperature (RT; 27°C ± 2°C). The influence of operating temperature on humidity sensing performance is examined in the temperature range of 15 °C–55 °C and a nearly linear relation (with Adj. $R^{2}$ of value 0.959) is observed toward change in temperature (35 °C–55 °C). Furthermore, the temperature sensing performance of the sensor is analyzed in a 15 °C–35 °C temperature range. The sensor displays good performance with a temperature coefficient of resistance (TCR) as −3.18%°C⁻ and −1.3%°C⁻ for 15 °C–25 °C and 25 °C–35 °C, respectively. Other important sensing characteristics, such as repeatability, hysteresis, response, and recovery times, are also analyzed and presented here.

I. Introduction

Humidity and temperature sensing is of significant interest in applications, such as environment monitoring [1], personal healthcare [2], [3], [4], [5], electronic skin (e-Skin) [6], [7], [8], food packaging [9], [10], [11], and agriculture [12], [13]. The regular and continuous monitoring of these parameters is important in the above applications. For example, most of greenhouse plants/crops require relative humidity in/around the range of 40%–80% [14], [15], [16] and optimal air temperatures in/around the range of 17 °C–27 °C [13], [15], [17]. This means that it is important to maintain the desired levels of humidity and temperature to ensure high growth rates of greenhouse crops/plants. This need for humidity and temperature measurements calls for cost-effective sensors, and as a result, a wide variety (e.g., capacitive, resistive) of temperature and humidity sensors have been explored [6], [7], [18], [19], [20], [21], [23]. However, complex synthesis, nonflexible form factors, and cost-ineffective fabrication hinder their widespread use. In this regard, printed sensors on flexible substrates offer an attractive route for flexible electronics applications [24], [25], [26]. The capability to choose the printing ink, substrate, and the flexibility of designing the electrode with desired pattern offers numerous opportunities to enhance the performance of these sensors. Furthermore, resource-efficient printing could improve the commercial viability of these sensors [27].